This invention relates to an electronic switching circuit for use in magnetic head drives and it is particularly useful in magnetic bias recording or erase circuits.
Prior art circuits for switching on and off currents applied to magnetic head windings such as for erase, bias or recording operations are known to utilize relay type electromechanical switches having machanical contacts connected between a head driving circuit and transducing windings. A control voltage is applied to the relay to control the switching. One such prior art circuit is employed for example for driving erase heads utilized in the audio portion of a video tape recorder, type VPR-2, manufactured by Ampex Corporation and described in VPR-2 Video Production Recorder, Catalog No. 1809384-02, page 10/9, issued in May 1980.
The above-mentioned type of switching circuit has significant disadvantages when utilized in transducer driving circuits where a high frequency noise signal is induced in the circuit. Every time when such switch is activated, sudden separation or connection of the mechanical contacts causes contact bounce and the noise potential which is present on the contacts effects considerable current spikes which enter into the transducing windings. For example, in multichannel recorders employing two or more adjacent recording channels high frequency crosstalk may be induced between the transducers of adjacent channels due to leakage flux originating from the transducing windings as well known in the art. If a first such transducer is for example in an erasing or recording mode a certain amount of crosstalk is induced from that transducer into the windings of a closely spaced second transducer. If thereafter the relay switch of the second transducer circuit is switched on by a control signal, as it is well known in the art, the crosstalk potential induced in the second transducer windings is shorted by the mechanical contacts of the switch thus effecting a current flow therein before the associated driving circuit is activated. Bouncing of the contacts follows accompanied by large spikes of current transient and ringing into the resonant circuit of the transducer, which undesirable oscillating current flows through the transducer windings. The above-indicated current causes unwanted erasure of existing information on the associated recording medium while recording thereon a sharp audible "pop" or "click" which is disturbing to the ear.